1. Field of the invention.
This invention relates to a ceramic welding process in which a mixture of refractory and fuel particles is projected from an outlet at an end of a lance in a gas stream against a target surface where the fuel particles combust in a reaction zone to produce heat to soften or melt the projected refractory particles and thereby form a coherent refractory weld mass. The invention extends to ceramic welding apparatus for projecting a mixture of refractory and fuel particles from an outlet at an end of a lance in a gas stream against a target surface where the fuel particles combust in a reaction zone to produce heat to soften or melt the projected refractory particles and thereby form a coherent refractory weld mass, and in particular to ceramic welding apparatus comprising a lance having an outlet for the discharge of a ceramic welding powder mixture.
Ceramic welding processes are principally used for the repair of worn or damaged refractory linings of furnaces of various types.
2. Description of the Related Art
In the ceramic welding process as commercially practised, a ceramic welding powder mixture which comprises grains of refractory material and fuel particles is projected against a refractory surface to be repaired in a carder gas stream which wholly or mainly consists of oxygen. The refractory surface is best repaired while it is substantially at its operating temperature, which may be in the range of 800.degree. to 1300.degree. C. or even higher. This has advantages in avoiding any need to wait for the refractory under repair to be cooled or reheated, so minimising furnace down-time, in avoiding many problems due to thermal stress in the refractory material due to such cooling and reheating, and also in promoting the efficiency of the ceramic welding reactions whereby the fuel particles bum in a reaction zone against the target surface and there form one or more refractory oxides while releasing sufficient heat to melt or soften at least the surfaces of the projected refractory grains so that a high quality weld repair mass may be built up at the repair site as the lance is played across it. Descriptions of ceramic welding processes can be found in British patent specifications GB 1330894 and GB 2110200-A.
It has been found that the working distance, that is the distance between the reaction zone at the target surface and the outlet of the lance from which the ceramic welding powder is projected, is of importance for various reasons. If that working distance is too small, there is a risk that the lance tip may enter the reaction zone so that refractory material is deposited on the end of the lance possibly blocking its outlet. There may even be a risk that the reaction could propagate back into the lance, though this possibility may be largely avoided by ensuring that the velocity of the carrier gas stream exiting the lance is higher than the speed of propagation of the reaction. There are also the possibilities that the lance may become overheated due to its close proximity to the reaction zone, and that it may contact the target surface again leading to possible blockage of its outlet. If, on the other hand, the working distance is too great, the ceramic welding powder stream will have an opportunity to spread out so that the reaction will not be so concentrated leading to a loss in efficiency, increased rebound of material from the target surface, a weld of less high quality, and even a risk that the reaction will fail.
The optimum distance between the lance outlet and the target surface will depend on various factors. For example, in a welding operation in which ceramic welding powder is discharged at a rate of between 60 and 120 kg/hr from a lance outlet having a bore diameter of 12 to 13 ram, such optimum distance is found to be between 5 and 10 cm. That optimum distance is rarely greater than 15 cm.
Because of the high temperatures typically encountered at a repair site, the target surface and other parts of the furnace lining tend to radiate strongly in the visible spectrum, and the reaction zone is itself highly incandescent. This renders direct observation of the lance outlet difficult, and this difficulty is increased as the length of the lance increases. Indeed lances with a length of 10 meters are not unknown, and nor is it unknown to perform a welding operation at a site which is out of direct view of the welding operator.